Abstract: We present our results on the Chandra X-ray Observatory Advanced CCD Imaging
Spectrometer (ACIS) observations of the bright Oort Cloud comets C/2012 S1
(ISON) and C/2011 L4 (PanSTARRS). ISON was observed between 2013 October
31-November 06 during variable speed solar wind (SW), and PanSTARRS was
observed between 2013 April 17-23 during fast SW. ISON produced an extended
parabolic X-ray morphology consistent with a collisionally thick coma, while
PanSTARRS demonstrated only a diffuse X-ray-emitting region. We consider these
emissions to be from charge exchange (CX) and model each comet's emission
spectrum from first principles accordingly. Our model agrees with the
observational spectra and also generates composition ratios for heavy, highly
charged SW ions interacting with the cometary atmosphere. We compare our
derived SW ion compositions to observational data and find a strong agreement
between them. These results further demonstrate the utility of CX emissions as
a remote diagnostics tool of both astrophysical plasma interaction and SW
composition. In addition, we observe potential soft X-ray emissions via ACIS
around 0.2 keV from both comets that are correlated in intensity to the hard
X-ray emissions between 0.4-1.0 keV. We fit our CX model to these emissions,
but our lack of a unique solution at low energies makes it impossible to
conclude if they are cometary CX in origin. We lastly discuss probable emission
mechanism sources for the soft X-rays and explore new opportunities these
findings present in understanding cometary emission processes via Chandra.